Reagent dosing system and method of dosing reagent

Abstract

A method of dosing a reagent into an exhaust gas stream of an internal combustion engine having an SCR catalyst, the method comprising injecting reagent from a reagent tank into the exhaust gas stream at a position upstream of the SCR catalyst using a reagent injector in accordance with a first dosing schedule in order to remediate a predetermined proportion of NOx in the exhaust gas stream, the first dosing schedule being associated with a first range of engine operating conditions; and injecting reagent from the reagent tank into the exhaust gas stream at a position upstream of the SCR catalyst using a reagent injector in accordance with a second dosing schedule in order to enable heat transfer between the reagent injector and said injected reagent, the second dosing schedule being associated with a second range of engine operating conditions.

The invention claimed is: 
     
       1. A method of dosing a reagent into an exhaust gas stream of an internal combustion engine having an SCR catalyst, the method comprising:
 injecting reagent from a reagent tank into the exhaust gas stream at a position upstream of the SCR catalyst using a reagent injector in accordance with a first dosing schedule in order to remediate a predetermined proportion of NOx in the exhaust gas stream, the first dosing schedule being associated with a first range of engine operating conditions; 
 injecting reagent from the reagent tank into the exhaust gas stream at a position upstream of the SCR catalyst using a reagent injector in accordance with a second dosing schedule in order to enable heat transfer between the reagent injector and said injected reagent, the second dosing schedule being associated with a second range of engine operating conditions; and 
 dosing in accordance with said first or said second dosing schedule in dependence on whether engine operating conditions lie within said first or said second range of engine operating conditions; 
 wherein the proportion of NOx in the exhaust gas stream which is remediated by dosing using said second dosing schedule is less than said predetermined proportion, and wherein said second range of engine operating conditions is characteristic of at least one of higher engine load and higher engine speed than said first range of engine operating conditions. 
 
     
     
       2. A method according to  claim 1 , further comprising:
 sensing the level of reagent in the reagent tank; and 
 halting the first dosing schedule when the sensed level of reagent drops below a predetermined level. 
 
     
     
       3. A method according  claim 1 , wherein the injector comprises an atomising dispenser and a positive-displacement metering pump, the method further comprising drawing reagent from the reagent tank by means of the metering pump and delivering it to the dispenser. 
     
     
       4. A method according to  claim 1 , wherein said second dosing schedule injects said reagent at a lower rate than said first dosing schedule. 
     
     
       5. An exhaust system for an internal combustion engine comprising:
 a common exhaust portion having a first end coupled to the engine for receiving an exhaust gas stream emitted therefrom and a second end which divides into first and second branches; 
 wherein the first branch comprises a first particulate filter, an SCR catalyst and an SCR doser for injecting reagent into said first branch at a location disposed between said first particulate filter and said SCR catalyst; and 
 the second branch comprises a second particulate filter, a Lean NOx Trap and a hydrocarbon doser for injecting hydrocarbons into said second branch at a location disposed between said second particulate filter and said Lean NOx Trap.